FTDNA now offers more markers (total of 37 in 2004 and 67 in 2006). The
below examples have not been revised to include those markers. In calculating
genetic distance, some of the new markers appear to be better described using
the "step-wise" method (exact difference), while for others the
"infinite allele" method (any difference counts as one) is
preferred, as discussed in the below examples. See http://www.ftdna.com/trs_gendist.html

In general, the genetic distance between two sets of results is determined
by subtracting the numbers and adding the resulting differences together,
treating all differences as positive (the "step-wise" method).
However, in some cases, the difference is reported as 1 (the "infinite
allele" method). In a perfect match of 12/12 or 25/25 there would be no
differences and the "total" would be 0. However, if there is not a
perfect match, the results for DYS numbers 389-1 and 389-2 and/or464 a b c d may need to be adjusted.

The below examples are a bit extreme since there is no close relationship
between the participants. However, they do provide examples of how DYS numbers 389-1 and 389-2 and/or464 a b c d are
adjusted. In the first example, the genetic difference appears to be 7 but
FTDNA reports it as 6; in the second example, what appears to be a difference
of 40 is reported as 30. The reasons for the exceptional counting rules for
these two markers are described at the links given below.

EXAMPLE 1 (389-1 and 389-2)

Locus

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

DYS #(1)

393

390

19*

391

385a

385b

426

388

439

389 1

392

389 2

458

459a

459b

455

454

447

437

448

449

464a

464b

464c

464d

Tom

14

23

16

10

14

14

11

13

10

12

12

28

Geo

14

23

16

10

15

16

11

13

12

13

12

29

7 =

-

-

-

-

1

2

-

-

2

1

-

1

6 =

-

-

-

-

1

2

-

-

2

1

-

-

The differences "add up" to 7 but because of 389-1 and 389-2 the
actual distance is 6. This is because the 389-1 count
is also included in the 389-2 count. In other
words, 389-2 has two parts: the 389-1 part and its other unique part. Therefore, the
calculation to determine distance subtracts the 389-1
count from the 389-2 for each person to
determine one set of numbers. Those results are subtracted and the difference
is added to the 389-1 difference.

389-2389-1
Tom 28 -
12
= 16

Geo 29 -
13 =
16
1
+ 0 =
1

For more information and examples see DYS 389i, 389ii MUTATIONS at Duerinck's website (section is
after the result charts).

EXAMPLE 2 (389-1, 389-2 and464 a b c d)

Locus

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

DYS #
(1)

393

390

19*

391

385a

385b

426

388

439

389 1

392

389 2

458

459a

459b

455

454

447

437

448

449

464a

464b

464c

464d

Pat

13

25

14

11

11

15

12

12

12

12

13

28

18

9

10

11

11

26

14

19

31

16

16

18

19

Geo

14

23

16

10

15

16

11

13

12

13

12

29

15

8

10

11

11

26

15

20

25

12

14

15

16

40 =

1

2

2

1

4

1

1

1

-

1

1

1

3

1

-

-

-

-

1

1

6

4

2

3

3

30 =

1

2

2

1

4

1

1

1

-

1

1

-

3

1

-

-

-

-

1

1

6

1

1

1

-

The differences "add up" to 40 but the genetic distance is
actually 30. This is partly because 389-1 and 389-2 contribute 1 (not 2) as explained above.
However, most of the discrepancy is because 464 a b
c d are unusual critters. In this example, they count as 3 (not 12 as it
"appears"). Why? Because no matter what the difference is in any of
the 464 a b c d groups, each counts only as 1
and any match in the group counts as 0 no matter what order it appears on
the chart. Therefore, in the above example, the two 16's count as a match. Thus, if you start by
ignoring the problematic numbers you would begin with a total of 26: